1. Field of the Invention
The invention pertains to a novel positive traction permeate tube and a method of making the novel positive traction permeate tube for utilization with spiral wound membrane elements used in pressurized membrane filtration, including microfiltration, ultrafiltration, nanofiltration and reverse osmosis. The invention pertains especially to novel compositions, structures and configurations for the positive traction permeate tube in a membrane filtration element and to devices and procedures for spirally winding a membrane element about a substrate permeate tube of stainless steel, or other suitable materials, cut to precise final length prior to winding and using positive traction during winding thus providing enhanced reliability of the finished spiral-wound membrane element. The novel positive traction permeate tube prevents system failure and destruction of the spiral wound membrane filtration element through breakage, channeling and unwinding thereof at higher fluid pressure drops and higher liquid feed flow rates associated with high viscosity retentates in membrane filtration processes.
2. Description of Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98
In a spiral wound membrane filtration element, a permeate tube is arranged at the center of the spirally wound membrane element. The permeate passes through the membrane on each side of the wound membrane leaves, then along the permeate carrier inside the membrane leaves and then through perforations in the permeate tube into the interior of the center of the permeate tube. It is through the center of this permeate tube that the permeate fraction can leave the membrane element. Thus the central tube in a spirally wound membrane filtration element is called a “permeate tube.” The permeate leaves the element via the permeate tube and the retentate can be passed on to another membrane filtration element to increase further the concentration of high molecular weight compounds in the retentate by extracting more permeate from the feed stream through a subsequent membrane element.
Further, for critical applications such as pharmaceutical and dairy operations, the permeate tube must be free from bacteria traps. Spiral wound membrane elements, such as those used in the dairy field for the separation of whey, are subject to a cleaning operation after periods of use, generally by the use of chemical cleaning solutions, such as caustics, detergents, chlorine, or combinations thereof, and are thereafter rinsed with clean water.
There are many different configurations for membrane filtration elements but the vast majority of membrane filtration systems have spirally wound membrane elements. Other configurations of membrane elements are tubular membranes, hollow fiber membranes and flat sheet membranes. Membrane filtration elements comprising spiral wound membranes and their construction are well known and are illustrated, for example, in U.S. Pat. Nos. 3,367,505, 4,033,878, and 4,792,401. A spiral wound membrane element generally comprises one or more laminate leaf assemblies of semipermeable membrane sheet and optionally other materials. These leaf assemblies are each arranged so that the feed liquid is carried over the membrane surface while permeate is carried through to a central permeate collection tube. The step of spirally winding the membrane onto a central tube is well known and demonstrated, for example, by Bray, U.S. Pat. No. 4,021,351.
Spiral wound membrane elements are a widely used membrane configuration in present day sanitary membrane filtration applications. Such membrane elements are installed inside elongated pressure vessels which may contain one or a series of up to six membrane elements or more. A high pressure feed stream enters the vessel through its inlet port at one end of the vessel and exits through its outlet port at the opposite end. The linear speed and high pressure drop is substantial, thereby posing a threat to the integrity of the membranes.
In the pharmaceutical, food & beverage, and biotech industries, contamination-free processing is critical. The integrity of the sanitary manufacturing equipment is essential for full compliance with the validation process. The potential for contamination increases with the introduction of peripheral components, such as filtration and temperature and pressure measuring instrumentation required to ensure process parameters remain within acceptable limits. As a result, these inline devices must themselves meet standards set by governing agencies to ensure there are no weak links in the sanitary chain.
The two most common of these standards include: (1) the “3A” standard promulgated by 3A Sanitary Standards, Inc. (3A SSI), a non-profit association representing equipment manufacturers, processors, regulatory sanitarians and other public health professionals. 3A Sanitary Standards and 3A Accepted Practices pertain to dairy and food processing equipment, and focus on sanitary design, materials, and surface finish; and (2) the “FDA-Approved” standard promulgated by the United States Food & Drug Administration (FDA), a federal agency that regulates all food processing and drug manufacturing in the U.S. It sets and enforces standards and government codes.
Most spiral-wound membrane elements now manufactured are rolled over a permeate tube made of plastic material, commonly polysulfone. In the prior art, a conventional torque force is applied in winding a spiral wound membrane element around a polysulfone tube. Plastic permeate tubes used in conventional manufacturing processes are longer than the tubes found in finished membrane elements as the original length of tube is cut to size after applying the spiral wound membrane to the substrate tube. A conventional spiral wound tube membrane filtration element is shown in FIG. 1. The wound element 10 involves a plastic permeate tube 11 having at least one notch 15 on one end for engaging a drive system, not shown, to wind the permeate tube 11 around which a membrane element 12 is wound under a conventional tension. The prior art permeate tubes are subject to mechanical slippage and breakage while winding the membrane element. Unlike the prior art, the permeate tubes of the present invention do not slip or break because they provide positive traction through an interlocking means, as will be discussed in greater detail in the Summary of the Invention.
The driving force required to roll the elements in a prior art permeate tube is applied to the ends of the permeate tube 11 by inserting a rolling machine chuck into the notch 15 cut at the end of the tube 11 as shown in FIG. 1. The notches 15 are eliminated with the excess length of the membrane element 12 and the plastic permeate tube 11 when the assembly is cut to its final length as shown in FIG. 2.
The entire assembly is cut at the ends, typically using a saw or other cutting means, to allow for placement in a membrane filtration vessel and so that the ends of the cut plastic permeate tube 11 may engage other elements in filtration vessel, by means of interconnectors or ATDs (Anti Telescoping Devices). Unlike the prior art, the invention does not cut the tube to length after wrapping but instead utilizes a tube of the desired precise length and positively wraps the membrane leaves to the final tube as will be discussed in greater detail in the Summary of the Invention.
For example, Bray, U.S. Pat. No. 4,021,351, teaches a membrane cartridge that is spirally wound around an injection-molded plastic permeate collection tube having transverse slits or tongue extensions at the end. The permeate tube can be made up of a plurality of interlocking plastic pieces mated with each other through the transverse slits or tongue extensions at the end of each tube segment. The membrane materials are then spirally wound upon the interlocking pieces of plastic permeate tube to form a membrane element.
In membrane filtration processes involving high viscosity retentate, the required higher fluid pressure drop and higher flow rates involved can cause channels to form between membrane layers while a filtration element is in use. This can occur at any time, and sometimes after only a few seconds of use under high pressure drop and high flow rate. “Channeling” is especially prevalent in spiral wound membrane elements for ultrafiltration of high viscosity retentate in partially filtered dairy liquids when the membrane element is wound about the permeate tube under conventional winding tensions, thus causing failure in the spirally wound membrane element while in use.
To avoid this failure, higher winding tensions are required in winding the membrane element to the permeate tube. However, when a membrane filtration application requires that the membrane element be rolled under higher than conventional winding tension, the prior art plastic permeate tubes tend to break, especially at the notches, thus causing unacceptable waste in the manufacturing process.
Changing only the composition of the permeate tube to stainless steel, instead of polysulfone, avoids the problem of breaking the tubes but introduces a new problem: cutting off the ends of the membrane element, including the stainless steel permeate tube, produces elements with too big variation of finished length as well as irregular end-faces of the tube. Cutting the stainless steel permeate tubes, of prior art design, before winding the elements leaves the tube without the slots at its ends and without positive traction. The driving force required for winding is reduced to whatever friction is available between the interior of the prior art stainless steel permeate tube and expanding chucks inserted at the ends of the tube and attached to the drive system of the rolling machine. This configuration results in slippage when the element is rolled under higher than conventional winding tension.
In conventional prior art tubes, failure due to slippage or breakage occurs at greater frequency as the winding tension is increased. Further, in the prior art little or no attention has been given to measuring torque or slippage in wrapping the membrane element and nothing known has been done to provide an accurate means of controlling and measuring torque or the tightness of the spiral wrap.
A positive traction substrate permeate tube of precise length and end-faces is needed that can withstand high torque and function under precise mechanical control that permits high tension winding in the production of a spiral wound membrane element without the substrate permeate tube slipping or breaking, and can withstand the pressure forces exerted on the tube while winding the membrane element and the high pressure drop and endure the high fluid flow conditions while in use in a filtration element.